Fabrication of mass-produced electronic items typically involves temperature- and atmosphere-sensitive processing. Conventional material deposition systems for electronic fabrication, including plasma-enhanced chemical vapor deposition PECVD and other vacuum deposition processes, rely on high temperatures and rigidly controlled ambient conditions. Conventional processes are typically subtractive, applying a conductive or other coating over a surface, treating the coating to form a pattern, then removing unwanted material. The conventional method for forming copper traces is one example of this process, requiring multiple processing steps with the use of toxic chemicals and the complications and cost of proper waste disposal.
Recent advances in printed electronics provide solutions that reduce the cost, complexity, and energy requirements of conventional deposition methods and expand the range of substrate materials that can be used. For printed electronics, materials can be deposited and cured at temperatures compatible with paper and plastic substrates and can be handled in air. In particular, advances with nanoparticle-based inks, such as silver, copper, and other metal nanoparticle-based inks, for example, make it feasible to print electronic circuit structures using standard additive printing systems such as inkjet and screen printing systems. Advantageously, nanoparticle-based inks have lower curing temperatures than those typically needed for bulk curing where larger particles of the same material are used.
Commercially available systems for curing nanoparticles typically employ heat from convection ovens or Xenon flash illumination energy. In such illumination systems, the Xenon lamps emit pulsed light that is directed onto films of nanoparticles to be cured. High light energy levels are required for nanoparticle curing. Exemplary nanoparticle-based inks such as Intrinsiq Material Ltd. CI-002, a copper nanoparticle based inkjet ink, or CP-001, a copper nanoparticle-based screen print ink, can be sintered through the use of photonic energy from Xenon lamp or other illumination, provided that the illumination system delivers adequate energy to volatilize coatings used in the ink formulations and to sinter and cure the inks over large surface areas.
Conventional approaches for conditioning of the nanoparticle material, however, suffer from a number of deficiencies. Xenon lamp emission is characteristically distributed over a broad range of wavelengths and often includes wavelengths that can cause unwanted effects, even at non-peak energy levels. This inherent spectral spread in Xenon lamp emission can have effects that result in incomplete or uneven curing. One result can be limited penetration of light energy into thicker films or premature sealing of top surface layers, trapping unwanted organic species in the remaining structure. This type of problem can occur when higher frequency light, such as light energy from the tail of the spectral distribution, inadvertently sinters the film and renders its top layers opaque to other wavelengths of emitted Xenon light, delaying or preventing curing of the lower layers. When this happens, the binder or organic suspension in which nanoparticles are suspended is only partially removed, causing uneven sintering, which can limit the conductivity of the applied materials.
With Xenon light, the distribution of energy intensity is non-symmetrical; the co-lateral dispersive energy that is produced can reduce curing efficiency or may even cause overheating and damage to the substrate. Further, pulsing of the Xenon lamp or other light source tends to create high energy peaks that can ablate films rather than melt and reflow films. As a result, the cured product may not have the desired structure.
Conventional methods are also limited with respect to the number of substrates that can be used. With materials having high thermal conductivity, such as aluminum, silicon, and ceramics, the applied energy intended for curing may dissipate too quickly. With such materials, heat can be drawn away from the area of incident light before sintering occurs. Furthermore, particular wavelengths emitted from the Xenon lamps can damage some polymeric films and other substrates, making them less suitable for curing.
Thus, it can be seen that there is a need for improved methods for sintering and curing nanoparticulate inks and similar materials.